Firefighters try to douse a fire Thursday at the United Nations' main relief agency in Gaza City.

Saeed Siam was killed "in the latest shelling on a house" in Gaza City's Sheikh Radwan neighborhood, said Al-Aqsa TV, which showed images of a body it identified as Siam.

Siam served as interior minister in the Hamas-led government before it was dissolved in 2007. He ranked behind only former Prime Minister Ismail Haniya and former Foreign Minister Mahmoud al-Zahar in Hamas' Gaza hierarchy.

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U.N. Secretary-General Ban Ki-moon is in the region as part of the diplomatic effort to secure a truce. He met with Israeli officials on Thursday, and condemned the attack on the U.N. Relief and Works Agency's compound in Gaza City that destroyed relief supplies and wounded three people. Watch as fire blazes at U.N. compound »

Speaking at a news conference in Tel Aviv with Israeli Foreign Minister Tzipi Livni, Ban said Israeli Defense Minister Ehud Barak said the shelling of the compound "was a grave mistake and he took it very seriously."

Israeli government spokesman Mark Regev said that Barak actually told the U.N. chief that "if it was Israel's fire, it was a grave mistake." Watch Regev respond to accusation »

Israeli Prime Minister Ehud Olmert expressed sorrow over the incident, but maintained that Israeli forces were responding to militant fire near the compound.

UNRWA Director John Ging denied there were any militants at the compound, and also said that at the time there was no fighting in the area.

Ging said UNRWA's headquarters -- located in a densely populated neighborhood -- was hit repeatedly by shrapnel and artillery, including white phosphorus shells -- the use of which is restricted under international law.